Electronic watermark detection apparatus and method

Abstract

Electronic watermarks in digital documents can be detected, even if the content of the digital documents has been subjected to distortion or other attempts to hide or destroy the watermarks. A method involves inputting the distorted image and comparison information, the comparison information including at least one of the original image or information used for embedding the electronic watermark; dividing a domain of the original image into a plurality of patches, based on the comparison information; inputting affine parameters of a predetermined patch from among the patches in the original image; extracting a patch candidate from the distorted image; using a predetermined electronic watermark detection method, judging whether the patch candidate in the distorted image adequately correlates with a neighboring patch in the original image; when the judging indicates an adequate correlation, outputting a part of the electronic watermark.

What is claimed is:  
     
       1. A method of detecting an electronic watermark in a distorted image that constitutes a distortion of an original image in which the electronic watermark is embedded, the method comprising: 
       inputting the distorted image and comparison information, the comparison information including at least one of the original image or information used for embedding the electronic watermark;  
       dividing a domain of the original image into a plurality of patches, based on the comparison information;  
       inputting affine parameters of a predetermined patch from among the patches in the original image;  
       on the basis of the affine parameters, extracting a patch candidate from the distorted image;  
       using a predetermined electronic watermark detection method, judging whether the patch candidate in the distorted image adequately correlates with a neighboring patch in the original image that neighbors the predetermined patch; and  
       when the judging indicates an adequate correlation, outputting a part of the electronic watermark obtained by the predetermined electronic watermark detection method.  
     
     
       2. The method of  claim 1 , further comprising: 
       updating the affine parameters to new affine parameters when the judging does not indicate an adequate correlation.  
     
     
       3. The method of  claim 2 , further comprising: 
       extracting a new patch candidate from the distorted image, on the basis of the new affine parameters; and  
       using the predetermined electronic watermark detection method, judging whether the new patch candidate from the distorted image adequately correlates to the neighboring patch in the original image.  
     
     
       4. The method of  claim 1 , further comprising: 
       storing the affine parameters as initial affine parameters for use in extracting new patch candidates when the judging indicates an adequate correlation.  
     
     
       5. The method of  claim 1 , wherein the judging includes: 
       subtracting pixels of the original image's patch from respective pixels of the distorted image's patch candidate; and  
       if a difference of the pixel subtracting is smaller than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       6. The method of  claim 1 , wherein the judging includes: 
       calculating pixel values of pixels in the patch before distortion, based on pixel values of a pixel in a determined domain in the distorted image;  
       calculating correlation values between the calculated pixel values and a random number sequence used to embed the electronic watermark; and  
       if the correlation values are larger than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       7. The method of  claim 1 , wherein the judging includes: 
       transforming into the frequency domain, the patch candidate in the distorted image and the patch in the original image so as to produce patch candidate frequency components and original image patch frequency components;  
       accumulating differences between respective patch candidate frequency components and original patch frequency components; and  
       if a result of the accumulating of differences is smaller than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       8. The method of  claim 1 , wherein the judging includes: 
       calculating values of pixels in the patch in the original image based on values of pixels of the patch candidate in the distorted image;  
       transforming into the frequency domain, the calculated values of the pixels in the patch in the original image, so as to produce original image patch frequency components;  
       calculating a correlation value between the original image patch frequency components and a random number sequence used to embed the electronic watermark; and  
       if the correlation value is larger than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       9. A method of determining a correlation between a predetermined patch of an original image in which is embedded an electronic watermark, and a patch candidate of a distorted image that constitutes the original image distorted by local distortion, the method comprising: 
       inputting the distorted image and comparison information, the comparison information including at least one of the original image or information used for embedding the electronic watermark;  
       dividing a domain of the original image into a plurality of patches, based on the comparison information;  
       inputting affine parameters of a predetermined patch from among the patches in the original image;  
       on the basis of the affine parameters, extracting the patch candidate from the distorted image;  
       using a predetermined electronic watermark detection method, judging whether the patch candidate in the distorted image adequately correlates with a neighboring patch in the original image that neighbors the predetermined patch; and  
       repeating the judging for all predetermined patches of the original image.  
     
     
       10. The method of  claim 9 , further comprising: 
       updating the affine parameters to new affine parameters when the judging does not indicate an adequate correlation.  
     
     
       11. The method of  claim 10 , further comprising: 
       extracting a new patch candidate from the distorted image, on the basis of the new affine parameters; and  
       using the predetermined electronic watermark detection method, judging whether the new patch candidate from the distorted image adequately correlates to the neighboring patch in the original image.  
     
     
       12. The method of  claim 9 , further comprising: 
       storing the affine parameters as initial affine parameters for use in extracting new patch candidates when the judging indicates an adequate correlation.  
     
     
       13. The method of  claim 9 , wherein the judging includes: 
       subtracting pixels of the original image's patch from respective pixels of the distorted image's patch candidate; and  
       if a difference of the pixel subtracting is smaller than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       14. The method of  claim 9 , wherein the judging includes: 
       calculating pixel values of pixels in the patch before distortion, based on pixel values of a pixel in a determined domain in the distorted image;  
       calculating correlation values between the calculated pixel values and a random number sequence used to embed the electronic watermark; and  
       if the correlation values are larger than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       15. The method of  claim 9 , wherein the judging includes: 
       transforming into the frequency domain, the patch candidate in the distorted image and the patch in the original image so as to produce patch candidate frequency components and original image patch frequency components;  
       accumulating differences between respective patch candidate frequency components and original patch frequency components; and  
       if a result of the accumulating of differences is smaller than a predetermined threshold, judging the adequate correlation to exist.  
     
     
       16. The method of  claim 9 , wherein the judging includes: 
       calculating values of pixels in the patch in the original image based on values of pixels of the patch candidate in the distorted image;  
       transforming into the frequency domain, the calculated values of the pixels in the patch in the original image, so as to produce original image patch frequency components;  
       calculating a correlation value between the original image patch frequency components and a random number sequence used to embed the electronic watermark; and  
       if the correlation value is larger than a predetermined threshold, judging the adequate correlation to exist.